Prospecting



4 Sheets-Sheet 1 4 JNVENTO ATTOBNEV mumusmarrok CHARLES r. TE/CHMANN GERI-IARD HERZOG Aug. 7, 1951 w. M.YSTRATFORD ETAL PROSPECTING Filed March 9, 1948 FIG.

Aug. 7, 1951 w. M. STRATFORD ETAL 2,562,961

PROSPECTING Filed March 9, 1948 4 Sheets-Sheet 3 FIG.

KNOWN FAUL 7' f lza KNOMV ORE BOOK LINES 0F EQUAL GAMMA RAY INTENS/TV PER UN/T MASSf/SORAD/N).

/ L mes TONE a, g #1 ISORAD/N MAP or SURFACE sunvsr USED TO LOCATE mun -0 sxmvs/o/v 40 OF OREBODY WHICH LIES BUR/ED IN V EN TORS ATTORNEY 8- 7, 1951 w. M. STRATFORD ETAL 2,562,961

PROSPECTING Filed March 9, 1948 4 Sheets-Sheet 4 7 BRANCH W" 66 M4P G M/1M DRAINAGE SYSTEM, ILLUSTRAT/NG UPSTREAM PROSPECT/N6 55 7D LOCATE SOURCE OF ANOMALOw GAMM4 X 90 90 I00 PAD/OACT/V/TY. SAMPLING POINTS ARE X MARKED "X." FIGURES ADJACENT SAMFl/NG 66 x POINTS ARE COUNTS PER SECOND PER 88 99 I /00 6445. OF SAMPLE. FORK BUR/ED OREBZDV 87 6.9

lSORAD/N FORK F FORK D 7 FORK "c" FORK B POINT I42 75 ROCKS //v THIS AREA AVE/PAGE 68 ATTQRNEV Patented Aug. 7, 1951 PROSPECTING William M. Stratford, New York, and Charles F; Teichmann, Mount Vernon, N. Y., and Gerhard Herzog, Houston, Tex., assignors to The Texas Company, New York, N. Y., a corporation of Delaware Application March 9, 1948, Serial No. 13,845

This invention is concerned with prospecting for mineral deposits, particularly those of metallic ores, and provides improvements which facilitate the location of such deposits.

Mineral prospecting is as old as mans use of metals, but despite its antiquity and the success ful application of geophysical methods in a few instances during recent years, it is still far more of an art than a science. Most of the large deposits of the base and precious metals owe their discovery to chance observation rather than to.

scientific surveys. Consumption of metals increases and ore reserves'decrease, thus increasing the incentive for discovering new deposits.

Nevertheless, discovery has not kept pace with depletion probably because the prospecting art has not kept pace with scientific development in other fields and is, in the majority of instances,.

inadequate for the location of ore bodies which do not disclose themselves through surface manifestations such as outcrops, gossan and the other ancient indicia employed by prospectors the world over. In a few special instances, such as pronounced magnetic, electrical or gravimetric anomalies, concealed ore :bodies have been discovered by geophysical'methods, but many ore bodies are not accompanied by such anomalies. Exploration by means of raises, cross cuts and other mine openings and by core or churndrilling is expensive and far from certain, for valuable ore bodies frequently are missed by a matter of a few feet. In short, there is a distinct need for improvements in ore finding. The instant invention supplies such need, at least in part.

As disclosed in co-pending application Serial No. 13,842, filed March 9, 1948, by Gerhard Herzog, it has been discovered that many orebodies,

whichmay or may not be radioactive themselvesare accompanied by detectable radioactive auras in the substantially barren country rocks in which they occur. By carrying an efficient gamma ray detector along a traverse in the general neighborhood of a suspected mineral deposit and determining the intensities of gamma rays emitted by the ground at different locationsalong the traverse, anomalies in gamma ray intensities which indicate the presence and location of the ore body may be discovered. The gamma rays detected originate in the barren country rock and they may appear at distances far in excess of the range of penetration of significant amounts of radiation originating in the ore bodyitself. Gamma ray surveys conducted as described above, both underground and on the surface, have successfully located ore bodies through as much as 2 Claims. (o1. 25083.6)

two hundred feet of barren country rock, which constitutes a barrier capable of absorbing any detectable amounts of even the hardest gamma radiation, r l

We have determined that the auras described above are also detectable by another method,

which is frequently more convenient, since it need not involve the movement of gamma ray detectors in the field to determine radiation intensities emanated from the undisturbed rock. Thus we have determined that gamma ray anomalies indicativeof the presence of ore deposits may be discovered by taking rock or soil samples atfa plurality of spaced points on or under the earths surface, measuring accurately theintensity of the radiation (particularly gamma radiation)" emitted by the individual samples and determining the intensity of radiation per unit mass (Weight or volume) of the individual samples."

These latter values are then related to the geome' try of the survey, for example by plotting them at the respective points on a map representative In general, weight represents sample mass more accurately than volume, but the latter may be" used when dealing with samples of approximately the same-specific gravity and screen'analysis."

V The invention may be applied .by sampling rock in'place or through'the investigation of; float samples. Thus detritus eroded from the deposit or its aura and transported-substantial distances frequently marks a trail which can be followed: to its source, as described in detailhereinafter:--

Although theoretically any type of gamma radiation detector might be employed in the prac' tice of the invention, provided that the observation time was sufl'lciently long to determine accurately the intensity of the gamma-radiation emitted by each sample, practical considerations require the use of a detector of high efiiciency forgamma raysseveral times that of the conventional Geiger-Mueller counter. The samplesare of necessity relatively small so that the source 'of' radiation is minute. In many cases, small but significant differences in intensity cannot be .detected at all with a Geiger-Mueller counter and other detectors of the same order of efficiency, no matter how long is =the period of observation, because the variations' in back'ground which occur from one observation period to another are greater than the difference in intensity to be determined. Even when the difference in gamma ray intensity between samples is relatively large, required observation times for each sample with a Geiger-Muellerdetector maybe a matter of days and hence'beyond 'all limits of practicality.

Fortunately,- suitable gamma ray detectors which permit accurate determination of the difference of gamma ray intensities between small samples have been developed. One such detector is described and claimed in .U. S. Patent No. 2,397,071, granted March .19, 1946. An even more desirable detector is that described and claimed in U. S. Patent No. 2,397,072. Both of these detectors are of the multiple plate cathode type and consist essentially of a stack of .perforated disks disposed coaxially and spaced parallel to each other, with one or more anode wires running through the perforations transverse to the disk surfaces. The large cathode area per unit of active volume thus obtained increases the 'efliciency for gamma rays several times, Without increasing efiiciency for the detection of background proportionately. Thus counters of this type have a gamma ray counting efficiency of 2.5% or more as compared with an efficiency of about /2 for the conventionalGeiger-Mueller counter.

All known detectors capable of detecting gamma radiation are also capable of detecting other radiation; including alpha and beta rays and cosmic radiation. This other radiation thus detected constitutes the background against which the gamma radiation intensity must be measured. Alpha and betafrayshave low pene trating power and the entry of'these rays into the counter from outside sources may be prevented by appropriate shielding. 'However, alpha and beta rays originating in the detector itself due to slight contamination of the materials of which the counter is made cannot be eliminated and contribute to the background. Cosmic rays also contribute to the background. Their gamma ray components may be prevented from contributing substantially to the background by adequate shielding, say several inches of lead or other high density metal, but the so-called pene-j trating particles of cosmic radiation are many times more penetrating than gamma rays and.

cannot be stopped with practical amounts of shielding, as, witness their occurrence" several thousand feet undergroundl In short, .it is possible to decrease but not eliminate background by shielding. f I

, The radiation which constitutes ,the background is emittedsporadically and at random. In the practice of the invention it .is, desirable to re-;

duce the background as much as practicable .in order to reduce error arising from thisfiuctuation as well as to increase contrast between measured intensities. The approach tov uniformity of background is also furthered by se-.- lecting aplace where the background isnaturally low and conducting comparative tests while the detector remains at that place protected by a constant amount of shielding. Thus, during a given survey, the detector should be keptat a. point, to which the samples ,arebrought-and should be protectedyon top, bottom .andsides :by adequate shielding, sayseveral inches of lead. If the survey is being conducted in a mining district an. underground location. inrock of low radio.- activity. and selected for 'it s 1o y/ background is de-,

.By examination of known mineral deposits in a given district in accordance with the invention,

it may be determined that a particular type of ..occurr.enc e in that district, say a copper-lead vein in granite, is accompanied by a particular type of anomaly, say .a positive one manifested by country rock in the neighborhood of the vein.

Once this fact is established, possibly by surveys conducted across a number of such known deposits that are cut by mine workings or which outcrop, prospecting proceeds in accordance with the invention, either underground or at the surface, in an attempt to locate similar anomalies which are not accompanied by anexposed ore.

body but may be indicative of a buriedone. In one aspect of. the invention the ratio of intensity of ,radiationto unit mass maybe supplanted by another ratio, namely that of the intensity of one kind of radiation to thein-tensity' of another kind :of radiation, both being emitted by the sample. of. alpha or beta radiation to. gammaor to each other may be determined, .andthese ratios em-.

ployed in much the samewayias the ratio. of. gamma ray intensityto .unit mass, this latterterm being employed throughout. .to. .meanx-both unit weight and unit volume.

Since gamma ray detectors in. general. do 'not detect with anything approaching 210.0% eff1.

ciency, the measurements .of-gamma rays which.

are .made in thepractice .of'the invention are comparative rather than-absolute, but thispresents. no obstacle. ifv detection efficiencies are .substane.

tially uniform from sample to sample.

The intensity .of the samples is of a low. orde and inthe case of the gamma radiation .the.ef.-.

ficiency of detection is. with practicable detectors substantially less than theefliciency forothertypes of radiation, sayi the penetratingportion of cosmic radiation. The gamma-radiation emitted in all directions. fromthe samples. in a preferred practice of the invention, the majority of the gammarays emitted by each sample are caused to enter the active yolume-of. a gamma, ray detector, for example, by employing a detector having an activevolume of annular cross,

section, the sample being disposed within the annulus. I

v These and other-aspects, tithejmgveimion will be clearly understood in the ,light of the fol-.

lowing detailed description of'. presently preferred practices, ,takenin-conjunction with the accompanyingdrawings in which: v I Fig. l is an elevatiompartly'in section, of a' preferred form of radiation detector'for the practi -oi e in n ion; v

Fig. v 2 is a horizontal section takenthrough the p ar tu f 'Fi 1 a e. t al n 2: 3 I Fig. 3 is a diagram illustrating heshielding of the detector. of Figs. land2;

Fig. 4 is a diagramqillustrating a, positive anomaly (indicative of the. existence of. an unknown vein) such, asmaybe determined by an underground survey, in accordancewith the in-v n i n;-

iafilis adi ramsimi ar to Eis-Ae c hat it. illus r es the. discovery 0.11. an unkn wn .021:

Thus, the ratio of. the; intensity assassin body. or mineral deposit through. the detection ofa negative anomaly;

Fig. 6 is an isoradin map illustrating a surface survey such as may be carried on to discover the extension of a known faulted ore body; and

Fig. '7 is a map illustrating the systematic sampling of a drainage system undertaken to dis-- cover an upstream mineral deposit.

-A toroidal or cup-shaped detector constructed in accordance with U. S. Patent No. 2,397,072, granted March 19, 1946, and particularly adapted to the practice of the instant invention is illustrated'by Figs. 1 andZ. It comprises a spaced several plates being in alignment to permit the passage through the plates of a plurality of tungsten anode wires l3. These wires are paral-- lel to each other and perpendicular tothe plates and are disposed respectively on the axes of the several rowsof holes. The wires are stretched taut between insulators I4, l5 at their ends and are connected in parallel with each other to a common conductor IS in plate form. A lead from' this plate passes through an insulator bushing I! and thence to a conventional counter circuit (not shown) including a D. C. high voltage supply, a pre-amplifier, an amplifier, a scaling circuit, and a recorder. Each cathode plate is provided with a notch to aid in alignment of the holes in the plates and a rib I8 fastened to the outside wall of the envelope passes through the several notches and prevents the plates from turning within the envelope. Spacers l9 are dis-' posed between the plates immediately inside the envelope to hold them apart in fixed relationship With each other, say on {a inch centers.

The top of the annular envelope is closed by an annular plate 20 extending from the inside wall of the envelope to the outside wall. The inside wall 21A of the envelope defines the side of a deep cylindrical cup 2! in which a sample to be investigated is placed. The bottom of the cup is defined by a plate 22. The bottom of the detector as a whole is closed by a cylindrical plate 23 through which the bushing I 1 passes. The outside wall of the envelope is threaded in its lower portion HA so that the envelope may be screwed onto a container holding a pre-amplifier (see Fig. 2).

The entire envelope is gas tight and is filled with a suitable atmosphere, say a mixture of alcohol and argon.

In operation, a high potential difference is established between the anodes and the cathodes, the potential difference being nearly, but not quite high enough to cause a discharge to take place. If an ionizing ray passes into the detector, a discharge may take place with resultant current flow, which produces a count. The discharge ceases after a short period'of time, after which the counter is again in conditionto count ionizing rays.

, The'toroidal detector is particularly desirable for'the practice of the invention sincev a ray Each cathode plate has a series of sym-- is; substantially surrounded by. active detector. volume and hence the registered intensityof its I radiation .tends to be increased.

In the practice of the invention it is important possible and be uniform from sample to sample in a given survey.v In order to bring about this :zthat the background be maintained as low as diminution in background and to. maintainit.

uniform, a site should first bechosen at which the background is low. Rock tends to stop cosmic rays and reduce the intensity of background from this source. Accordingly, especially in surveys in known mining districts, it may be convenient to place the detector underground, for example, in an abandoned stope, drift or crossout. In choosing the location, a preliminary survey should be made with a detector having a high efiiciency for the gamma rays emanated from the ground. When an underground space;

say a stope, is found in which the background intensity is at a minimum, the toroidal detector" is set up as shown in Fig. 3. Thus the toroidal detector is mounted above the preamplifier and both are disposed in a lead shield having a thick bottom, thick walls, and a removable cover', like-' wise lead. Generally speaking, the thicker the lead the better, although a shield of two to six inches is sufficient. It is convenient to build the bottom and the walls of the shield with lead blocks of aweight which may be handled con veniently. As shown, the shield and the detector of Fig. 3 are disposed in a cave with a substantial thickness of rock, preferably several hundred feet, above the detector in order to reduce the".

effectof cosmic radiation as much as possible. To consider a typical underground survey conducted in accordance with the invention, say

along a cross out which 'is suspected of [being near an ore body, representative samples are taken at spaced intervals, say 20 fe'et,'from the These samples should be sufficiently large to be representative. Samples of a kilogram or more are" rock along one side of the cross cut.

recommended.

The individual samples, are kept separate and I each is crushed to approximately the same maximum size and size distribution. 7 Crushing to minus 1Q mesh is desirable and even finer crushing may give better results. After the samples have been crushed a, representative portionbf each is placed .in the counter. I A thin walled glass test tube 'may be employed to hold the" sample and the test'tube dropped into the cup of thetoroidal detector.

Samples of grams have been 'used with f good results, although generallyspeaking, the In any case, each" vention, it is essential to know the background count. Consequently, with the detector empty' the background is measured to discover its intensity, which varies sporadically within limits and may alsovary with the time of day. This latter variation is known as the diurnal variation and should be determined by taking measurements of; the background. intensity at intervals during: The background, count should be:

the day. checked frequently, at least once a day.,'

err h b c g o nd s bee -e abli h ,each; sample in turn is placed in the detectorgand;

left: there until'a-fixednumber of countszhasr' The corrected count, i. e. the. total count minus.

the-backgroundis now divided by the observation time to giveta value ,for gamma ray intensity, i. e. counts per unit time. This result is inturn divided; by the weight or volume of thesample, prefably the weight, to give a figure for intensity per unit mass. The values thus determined are plotted along a longitudinal section of the cross cut along which the survey is made.

Fig. 4 illustrates such a survey along a cross cut, in limestone. A known vein of copper ore is cut :by the cross ,cut and it is suspected that another similar vein has been faulted off so that ity is not penetrated by the cross cut. A survey along the cross cut is made by taking samples at spaced points as shown. Each of these. samples is; then. treated as described above to determine the intensity per unit mass of gamma rays emitted by the sample and'the valuesthus determined areplotted as shown. The known vein of copper ore is associatedwith a strong positive anomaly. This anomaly is apparent in the country rock' on both sides of the known vein and as the vein is approached. from. either side, the, intensity of gamma rays increases. In the unaltered limestonewell' removed from. the ,vein, the intensity is relatively low.

On the right side of the plot of gamma ray intensity along the'cross out there is another positive anomaly less marked than that associated with the known vein, but still sufficiently strong to' warrant investigation, say. by cross cutting or drilling, In other words, this positive anomaly may indicate the. presence ofa hitherto, unknown ,ore body. Since valuable "ore bodies frequently are missedby a matter of a few feet, this clue to location of such-an ore body' is; important. I

. A noma1ies detected inaccordance with the in-- stant invention may be either positive or nega:

tive. Negative anomalies are sometimes associated with zinc sulphide orebodies in. quartzite and a survey for the detection-bf such a-negative, anomaly is illustrated in Fig. 5. In this case a known veinof zinc orewhich manifests a negative anomaly crossed bya-cut in a country-rock of quartzite. The presence of this negative anom aly. associated'with a' vein, of, known zinc ore is detected by taking samples along the cros cut at spaced intervals as shown and the survey is continued to the face of the cross cut shown -,at the right.- The intensities; ofthey unaltered quartzite samples are 'relatively'high; but as the unknown vein'is approached these intensitieside crease; reaching a minimum inthe neighborhood oi -the -vein itself. This indicates that the-cross cut-should be extended; and that a mineralizedzone possibly is being-approached. 7

Surveys similar to those of; Figs. -4- and 5 maybe made by examininga series of core samples from-known locations in-a bore hole, even' those of such small size that;logging byypassinga' radiation detector 1 along the -bore-would be im- P actical. Y r i A surface survey to. determine the-extension. off a known ore body which has beenxlost'throughi.

faulting is illustratedin themap of Fig. 61- The known: ore; body .is indicated on the upper left- This ore. body-does not outcrop andends at a fault. It .hasbeen mined out.

. The country rock for the'most part isqu artzitq; but contacts with limestone occur in the;lower..

left.hand; corner, .wherea fault-and a contact mnmeet.

There is reasontobelieve; that there is. an extension of'thelknown ore body in the general; neighborhood. In. an effort :to discover this,ex--- tension, thesurvey ismade by taking rock samples at spaced known locations on. the surface of the ground in the general vicinity. These samples are-treated asin thecase of the underground. surveys and the gamma ray intensities per unit mass-for eachsample are plotted on the map at -points corresponding to those from which the samples came. After these intensities have been plotted on the map; lines of equal intensity (which. we callisoradins) are plotted on the map as.

shown. The isoradins above theknown ore body disclose apositiveanomaly and form a recognizable. pattern. A similar isoradin patternoccurs below and to the right of the known ore body in Fig. 6,. thus indicating the position of the faulted portion or extension of the ore-body.

The. isoradins crossvthe fault in the quartzite. in a smooth. fashion,,but at the contact between. the limestone and the quartzite, the isoradins. are interrupted and show .the lines along which these dissimilar rocks meet.

Fig. '7 illustrates systematic samplingv of a. drainage system proceeding. upstream tolocate;

sources of anomalous. gamma ray intensities found inthe sands of a-stream bed or in suspended solids carried by the stream some distance ,be1ow. a buried orebody. In the process illustrated by Fig. '7, sampling is begun at a point A. downstream inthe drainage basin. Therocks.

in this. basin: showanmzerage gamma ray intenan intensity of .75 per minute per 100 gms. Proceeding upstream va value of '76 is noted. Since both of these are above the average for the rockv in the.area-,.there is an indicationthat material '50. of highergamma ray intensity has been carried into the stream, from above, possibly by erosion of the radioactive aura from an orebody which. is itself. not. yet exposed by erosion. When the. first fork B inthestream. is attained, samples, The intensity of are taken on both branches.

the sample in the left hand branch. is low, indicating, that thematerial of. high gamma intensity is transported by the right hand branch in which an intensity ratio, of '76 is noted. Consequently 60. the right handv branch is followed. At the next fork C a similar difference in intensityratios is noted from the samples of the respective branches and that giving the highest intensit is followed,

this procedure continuing upstream toward the 65. head waters past forks D,v E, F- and G. Finally; as'theselected branchI-I is followed upstream an intensity of counts per minute per gms. is noted, butfurther'progress upstream gives lower intensities indicating. that the mate- Io. rial of high intensity is being washed into; the

stream below thesepoints of low intensity; Thestream bed is nowleft and samples are taken uphill along the course that float from' an out: cropping radioactive aura would necessarily take. 75 Eventually,- an area-is reached inWhich aclosed 'isoradiriof sa'y:120"counts per minute per 100 gmsj can be plotted; with- 'lower' intensities away fromthis'isoradin-a nd higher intensities within it. This is a positive anomaly which may be indicative -f an ore" body under the surface in the neighborhood of theclosed isoradin. The positive anomaly sought having been discovered, a detailed examinationsimilar to that illustrated in Fig. 6 can be harried out, this to be followed ,by drilling or sinking if=the results seem to -justifysuchastep. l-f.

The method of; the invention is adapted to the reconnaissance of large areas, by taking samples of rocks from the surface at random known points or at the intersections of a grid laid. out 1 in Fig. '7 proceeding upstream with a view toward determining the site from which the high intensity rock is being eroded.

In the application of the invention to the investigation of a drainage system, the location of both positive anomalies and negative anomalies may be sought by sampling upstream. If a downstream sample shows a low intensiity of gamma rays per unit mass as compared with the average intensity per unit mass of gamma rays emitted by the rocks of the drainage basin, the procedure is the same as that described in the case of Fig. '7, except that the direction of low intensity, as indicated by successive sampling at forks, is pursued upstream.

Samples taken in the practice of the instant invention may emit alpha and beta rays in addition to gamma rays. In some instances, the ratios between alpha, beta and gamma intensities emitted may be significant and the invention contemplates the determination of one or more of these for a series of samples.

Alpha rays have very little penetrating power; beta rays have higher penetrating power, but not nearly the penetrating power of gamma rays. This difference in penetrating power of the rays may be employed to determine the significant ratios of intensities. By way of example, the inside wall of the envelope detector of Figs. 1 and 2 may be made very thin so as to be transparen to alpha radiation. Such thin walls or windows require that the pressure of the gas within the envelope be approximately the same as the pressure of the gas outside the envelope, or that the window be reinforced.

Relative intensities of alpha rays emitted by samples are found by placing the sample directly in the cup of the thin walled detector and determining the count as described previously. Thereafter a shield which cuts out alpha rays but is transparent to beta and gamma rays is placed in the cup conveniently by placing the sample in a container (such as a glass test tube) having a wall of such thickness that it will stop alpha rays but will permit beta and gamma rays to enter the counter. The count is determined under these conditions. Lastly, the sample is transferred to a second container which has a wall so thick that it efiectively stops alpha and beta radiation while permitting the gamma ray radiasample.

.910 tion to enter the counter. Acount is obtained with the sample in the counter under these conditions. The background is subtracted from each of the three counts described above. Alpha intensity is determined by subtracting the second count from the first. Beta intensity is determined by subtracting'the third count from the second. The third'count minus background gives the gamma intensity.

In summary, the process is conducted in three stages. In the first stage the counter is made transparent to alpha,=beta and gamma rays. In the second stage, wallthickness is increasedto eliminate alpha while permitting beta and gamma to enter. In the 'thirdstage the wall is transparent only to gamma.

An alternative method for determining alpha intensity involves the investigation of radioactive gases (i. e. radon, actinon or thoron) emitted by the samples. T determine the intensity of alpha rays emitted, for example by radon, the rock sample obtained inthe exploration is crushed under vacuum and gas is' extracted from the This gas may be fius'hed'directly into the detector itself, for example, by' a stream of the gas filling (say argon and alcohol). In this way the alpha rays are emitted'withinthe envelope and the problem of penetration is effectively solved. As in the previous cases alpha intensity is determined in terms of unit mass.

After the radon has been extracted from the sample, the latter is subjected to further tests in the toroidal detector employing first a wall which is transparent to beta and gamma, thereafter a wall which is transparent only to gamma.

In some cases, the pertinent ratio may be between alpha or beta and gamma and in other cases between alpha and beta. In all such cases the ratios of intensities per unit mass are determined and plotted in the same way as intensities in the operations illustrated in Figs. 4, 5, 6 and 7. In the case of a mapping operation, lines of equal ratios are drawn in the same fashion as the isoradins, to aid in interpretation of results. In the case of a traverse, say along a mine working, the ratios are plotted as ordinates along the traverse as in Figs. 4 and 5.

The treatment of wet samples obtained in prospecting with the method of the invention, for example, along a stream course, may be conducted in various ways. The simplest fashion is to take samples of the sand in the stream bottom or a sample of the water itself bearing suspended solids. These samples are then evaporated to dryness and the procedure thereafter is as described for solid rock samples, except that usually no crushing is necessary. In evaporation of the samples to dryness however, sources of alpha radiation usually are lost, so that if the determination of alpha radiation intensity is important the radioactive gases (say radon, actinon or thoron contained in the wet samples) should be extracted by vacuum and subjected to detection prior to drying the sample.

In desert areas or on steep terrain on which water flow is irregular deposits may be traced to their source through the practice of the invention by conducting a sampling program uphill along the course of the float from an outcrop of a radioactive anomaly in the country rock.

Percolating ground waters may also carry dissolved or suspended raiodactive material from a mineral deposit or from its aura and it is within the contemplation of the invention to sample the ground water of an area being prospected by layswam-em ducting. apreliminarysurvey"by, determining thelntensity of background radiation.at-a.plurality of: locations and, selecting: that location. at which the background-intensity is the lowest andjdetecting; the comparative: intensitiessof. gamma: rays emitted bypeach" sample by, determining.- the respective sample masses and; at theselected location;' detecting the intensity of gammaradiation emitted" by each samplev together with theintensity; of thebackground radiation, and subtracting the separately-determined:background-radiation from the intensity measurements made for therespectivezsamples.

2.; Process; according; to..- claim L 1m. which; theintensity :for; eachasampleis determinedwhilejhe sample. is: substantially, surrounded: by: counter volume.

WILLIAM M. STRATEQRD; CHARLES E. TEICHMANM. GEBHARD:HERZ.QG.

REFERENCES CITED;

The following references. are of record in the file of this patent:

UNITED STATES 1 5mm Number. Name Date 2,330,829.- Lundberg.. Oct. 5., 19.43 2,350,967. Langer .E"eb. 8,1944 $397,072 Hare .,Mer. 19., 1946 OTHER; REFERENCES Heiland: Geophysical Exploration, Prentice Hall, Oct: 19.40; p 870-875;

. Evensetiali Review of Scientific Instruments, vol. 1'0',,Nov. 1939', 332-336.

L'ocher; and Weatherwa'x: Radiology, vol. 2'7, I936, pp. 149-15.?I 

